Discrete sacrificial anodes have been embedded in cavities in concrete to protect the reinforcing steel. In the process the sacrificial metal element dissolves and a protection current flows from the anode to the steel. A backfill is a material surrounding the sacrificial metal element of the anode that maintains an electrolytic contact between the electrolyte in the surrounding environment and the surface of a sacrificial metal element. In anodes for reinforced concrete, the backfill will also contain an activating agent that maintains anode activity. An anode is an electrode that supports a net oxidation reaction on its surface such as the dissolution of a sacrificial metal element in the case of a sacrificial anode. To protect the steel, electrons must flow from the anode to the steel. This electron movement may be promoted by the presence of a power supply between the anode and the steel. The electron movement will primarily occur in electron conducting conductors. Furthermore ions must move through the electrolyte between the anode and the steel. Positive ions will move from the anode to the steel when the steel is protected. A flow of both electronic and ionic current occurs in the process of protecting the steel.
One commercially available sacrificial anode assembly based on WO 94/29496 comprises a zinc metal element activated by hydroxyl ions in a porous material that surrounds the zinc. The zinc corrodes to form soluble products that precipitate out in the pores of the surrounding material. The anode and backfill are pre-formed as a rigid unit. The unit is subsequently installed in a cavity that is formed in a concrete structure. An embedding mortar, which will typically be a cementitious repair mortar, is used to fill the space between the unit and the concrete surface of the cavity. This mortar fixes the unit in place and provides a path for electrolytic contact between the electrolyte in the backfill and the electrolyte in the surrounding concrete.